A biaxially oriented polyethylene terephthalate (hereinafter referred to as "PET") film has heretofore been widely used as a support in a magnetic recording medium because of its good surface smoothness and mechanical characteristics, and a magnetic recording medium using it as the support is excellent in abrasion resistance and running properties. In recent years, high density recording and long time recording have been increasingly required for video tape using a polyester support. Accordingly, an increasing demand exists for the support to possess excellent smoothness and sliding properties, and further to be thin in thickness yet demonstrate high strength and excellent running durability.
In order to meet the above requirements, use of a PET support, which is increased in strength in both longitudinal and width directions, has been made. However, with a reduction in the thickness of the conventional PET support, the strength of the support becomes insufficient and thus deformation or damage of a tape edge during running system occurs, seriously deteriorating recording and reproducing characteristics.
In order to increase the strength while decreasing the thickness of a support tape, JP-A-62-234233 (the term "JP-A" as used herein means an "unexamined published Japanese patent application) discloses the use of polyamide, and EP-A-229346 discloses the use of PEN. The support made of polyamide or PEN is increased in strength because of strong internal orientation of the material, but the tear transmission resistance of the support is seriously low because of its strong internal orientation. For this reason, at steps of coating, calendering and slitting of the magnetic recording material, cutting often occurs and the yield is seriously decreased. Even though the magnetic recording material can be fabricated into the desired tape, if the tape is damaged, even though slightly, as a result of troubles in the running system, the tape is easily cut, which will become a serious problem for the quality of the product.
JP-A-63-197643 and 63-212549 disclose composite polyester. If, however, a proper lamination ratio is not employed, a serious reduction in tear transmission resistance and insufficient strength are produced by the strong internal orientation as in the above case. Thus, as steps of coating, calendering and slitting of the magnetic recording material, cutting is often caused, leasing to a considerable reduction in the yield, and furthermore the deformation or damage of the tape edge in the tape running system is caused, leading to a serious deterioration of recording and reproducing characteristics.
JP-A-63-197643 discloses a composite polyester film, which is a three layer composite film composed of biaxially oriented polyethylene-2,6-naphthalene carboxylate films as outer layers and a biaxially oriented polyethylene terephthalate film as a core layer as sandwiched between the two outer layers, and in which the thickness of the two outer layers is 1/40 to 1/5 of the total film thickness.
In the above composite polyester film, it is attempted to prevent deposition of oligomer crystals which are responsible for dropout, by employing the above construction. However, in the conventional balanced type, if the above construction is employed, the strength in the longitudinal direction (lengthwise direction) and the strength in the width direction (lateral direction) are not sufficiently high and edge deformation is caused, leasing to a considerable decrease of output.
Although PEN is known as a material of very high strength, its tear strength in the lateral direction (width direction) is small and is not at the level that can be employed for practical use.